1. Field of the Invention
The present invention relates to an optical system and more particularly to the measuring of a flare in an optical system.
2. Description of the Related Art
A projection exposure apparatus has been conventionally employed to fabricate a micropatterned semiconductor device using photolithography. The projection exposure apparatus projects and transfers a circuit pattern formed on a reticle (mask) onto a substrate such as a wafer by a projection optical system. In recent years, as micropatterning of semiconductor devices advances, a demand for the line width uniformity of a pattern transferred onto a wafer is becoming stricter. Along with this trend, degradation in line width uniformity attributed to a flare generated in the projection optical system, which falls within the conventional tolerance, may become an issue. The flare means herein stray light which adversely affects reticle pattern imaging.
Flares generated in an optical system such as a projection optical system are roughly classified into a local flare attributed to forward-scattered light generated on the surface of an optical member (e.g., a lens) or a coating film on an optical member, and a long-range flare attributed to light reflected by a coating film on an optical member. A flare that degrades the line width uniformity is a local flare, which will be referred to as a flare hereinafter.
The Kirk method, for example, is known as a flare measurement method (see “0.85NA ArF Exposure System and Performance”, Proc. SPIE, Vol. 5040, pp. 789-800, 2003). The Kirk method as a flare measurement method will be explained with reference to FIGS. 10A, 10B, and 11A to 11D. FIGS. 10A and 10B are views showing a measurement pattern MP used in the Kirk method, in which FIG. 10A is a schematic top view of the measurement pattern MP, and FIG. 10B is a schematic sectional view of the measurement pattern MP. FIGS. 11A to 11D are schematic sectional views showing a resist pattern formed on a substrate by illuminating the measurement pattern MP with exposure light.
In the Kirk method, a measurement pattern MP having a box pattern BP made of a light-shielding film and a clear field CF which surrounds the box pattern BP is illuminated with exposure light to form a resist pattern on a substrate, as shown in FIGS. 10A and 10B. At this time, in response to a change in the exposure amount (exposure time), the sectional shape of the resist pattern corresponding to the box pattern BP formed on the substrate changes as shown in FIGS. 11A to 11D. Using this effect, the exposure amount is increased to obtain an exposure amount α when any resist other than the resist pattern corresponding to the box pattern BP is removed (FIG. 11B), and an exposure amount β when the resist pattern corresponding to the box pattern BP disappears due to the influence of flares (FIG. 11D). Then, the ratio of the exposure amount α to the exposure amount β (α/β×100 [%]) is determined as a flare.
Another known method measures flares with different scattering distances using a measurement pattern having a plurality of different patterns. Still another known method measures a flare by detecting an aerial image (optical image) of a measurement pattern by a sensor, instead of illuminating a measurement pattern with exposure light to form a resist pattern on a substrate.
A method of measuring the wavefront aberration of the projection optical system, and calculating a flare attributed to the wavefront aberration has also been proposed (see “Full optical column characterization of DUV lithographic projection tools”, Proc. SPIE 2004, Vol. 5377, p. 1960, and “New paradigm in Lens metrology for lithographic scanner: evaluation and exploration”, Proc. SPIE 2004, Vol. 5377, p. 160). More specifically, a flare is calculated by inputting the measured wavefront aberration or the PSD (Power Spectral Density) or PSF (Point Spread Function) obtained from the wavefront aberration to an optical (imaging) simulator.
This method can obtain information concerning the flare scattering distance and directionality from one wavefront aberration (i.e., the wavefront aberration of the projection optical system on one measurement surface). This method also can calculate flares generated under various kinds of exposure conditions including, for example, the reticle pattern and the illumination condition. The limit of the computable flare scattering distance depends on the spatial resolution with which the wavefront aberration is measured, and the spatial resolution mainly depends on the pixel size of a sensor which senses an interference pattern (interference fringes).
Unfortunately, to measure flares generated under various kinds of exposure conditions, the method of measuring a flare by illuminating a measurement pattern with exposure light and the method of measuring a flare by detecting an aerial image by a sensor perform flare measurement for each set of the exposure conditions, resulting in an increase in the measurement load. In addition, to obtain information concerning the flare directionality, these methods perform a larger number of times of measurement.
In the method of calculating a flare from the wavefront aberration, the measurement limit of the flare scattering distance depends on the measurement limit of the spatial frequency of the wavefront aberration. This is because the flare scattering distance is proportional to the spatial frequency of the wavefront aberration. To obtain a flare with a long scattering distance, it is said to improve the spatial resolution with which the wavefront aberration is measured and to reduce deterioration in high-frequency components or high-frequency measurement noise in a measurement apparatus which measures the wavefront aberration. Despite this common view, the result of close examinations conducted by the inventor of the present invention proved that even when the wavefront aberration is measured while ensuring a sufficiently high spatial resolution and reducing deterioration in high-frequency components, only one high-resolution wavefront aberration is insufficient to obtain a flare with a long scattering distance (20 μm or more) with high accuracy.